There are many reasons why languages become extinct, but to pick an extreme example: a couple of thousand years hence, after the apocalypse, the only present-day language still being spoken then is, say, Portuguese. But there’s all this written material from the lost cultures that you, the post-apocalyptic survivor, want to decrypt. Technology is totally different by then - except optical magnification, which remains fundamentally similar to how it has been since humans began using it millennia before. In the ancient ruins of Fort Mason, San Francisco, you find a Rosetta Disk, successfully engineered to remain undamaged by fire and water and air and time. Around the edges of the disk, there’s writing large enough for you to read; but you see there are more small markings on it. You put the disk under a microscope. You see text you recognise in Portuguese - huh, that text next to it is similar in size and shape, you start spotting a word that appears with similar frequency as in the Portuguese, thus you deduce what that one means, and then another, you start seeing linguistic patterns and gain some insight into what characters and writing system are being used. And if you stick with the task long enough, you figure out that language.
This isn’t some futuristic dream. It has already happened. Most famously with the Rosetta Project’s namesake, the Rosetta Stone.
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